The present invention relates to a golf ball material containing an ionomer composition made up of, in the molecular-weight distribution, three distinct components. The three-component ionomer composition includes an ethylene-α,β-unsaturated carboxylic acid-α,β-unsaturated carboxylic acid ester terpolymer in which at least some portion of the carboxylic acid functional groups is neutralized.
Ionomeric resins (ionomers) are useful materials, especially in golf balls, among other things. In particular, terpolymer ionomers obtained by neutralizing with metal cations terpolymers of an α-olefin such as ethylene, an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, and an ester (softening monomer) of any of these unsaturated carboxylic acids are useful as materials capable of imparting softness to golf balls.
Golf balls manufactured using ionomer materials generally have improved rebound resilience, toughness, and durability as compared with a balata ball construction, and are thus structurally important materials in golf balls. Ionomers such as Surlyn® available from DuPont and Iotek® available from Exxon-Mobil Chemical have become the materials of choice over traditional balata (trans polyisoprene, natural or synthetic) rubbers. However, ionomers having a high durability generally tend to be very hard. When used as golf ball materials, the resulting balls lack the softness required to impart the spin necessary to control the ball in play and also tend to have a poor feeling on impact.
Yet, there has been a desire among golfers for a golf ball endowed with properties that combine the good impact resistance and distance of hard copolymer ionomers such as ethylene-(meth)acrylic acid copolymers with the soft feeling and spin provided by balata.
Terpolymer ionomers obtained by neutralizing with metal cations a terpolymer composed of an α-olefin such as ethylene, an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, and an ester of such an unsaturated carboxylic acid (softening monomer) can provide softness. However, such terpolymer ionomers are not suitable by themselves as golf ball materials because of a low rebound resilience.
In order to prepare ionomeric materials which provide softness, high rebound resilience, high durability and spin control, as related in a number of published U.S. patent publications (Patent Documents 1 to 3 below), melt blends of hard ionomers and soft ionomers have hitherto been investigated. However, compared with hard copolymer ionomers by themselves, blends which contain also a soft ionomers tend to have a poor scuff resistance when used in golf ball covers. In addition, a number of U.S. patent publications (Patent Documents 4 to 7 below) describe materials obtained by using a high-molecular-weight highly neutralized ionomer together with a low-molecular-weight higher aliphatic acid metal salt so as to improve scuff resistance. However, in contrast with a high-molecular-weight ionomer component used by itself, formulating a low-molecular-weight higher aliphatic acid metal salt gives rise to incompatibility problems. As a result, when such a blend is used in certain layers of a golf ball, the golf ball will have a poor durability. Moreover, a high degree of neutralization gives rise to a decline in the flowability of the material, which adversely affects processability when the golf ball is injection molded.
Also, as illustrated in several U.S. patent publications (Patent Documents 8 to 10 below), polyurethanes have been developed in the past as golf ball materials. Polyurethanes, particularly when used in golf ball covers, are materials which provide a combination of a good scuff resistance, spin control and a soft feeling on impact. However, because polyurethanes have a low rebound resilience, they reduce the performance of the golf ball. Moreover, thermoset polyurethanes are more difficult to process than thermoplastic ionomers and cannot be recycled, increasing the material costs.
Patent Document 1: U.S. Pat. No. 4,884,814
Patent Document 2: U.S. Pat. No. 5,120,791
Patent Document 3: U.S. Pat. No. 5,971,871
Patent Document 4: U.S. Pat. No. 6,100,321
Patent Document 5: U.S. Pat. No. 6,653,382
Patent Document 6: U.S. Pat. No. 6,777,472
Patent Document 7: U.S. Pat. No. 6,815,480
Patent Document 8: U.S. Pat. No. 6,974,854
Patent Document 9: U.S. Pat. No. 7,041,769
Patent Document 10: U.S. Pat. No. 7,090,798
Patent Document 11: JP-A 2003-512495